Actually, I'm starting to wonder if this should be an option you can pass
as a syncrepl (really libldap) configuration directive. There's something
to be said for having aggressive-ish keepalives on server-to-server
communication (with, hopefully, good bandwidth/latency/communication
costs/etc.) and keeping your site defaults for "normal" clients. OpenLDAP
already inched in this direction with the ability to set different
idletimeout, etc., for the syncrepl client. Is anybody interested in
this? Maybe I'll do it (or have a student do it) if a few people think
it'd help...

I'm guessing you haven't read the syncrepl section of the man pages in a
while. See the "keepalive" option.